Lithium-ion batteries have known a phenomenal technical success and commercial growth since the initial work by. Sony in the early 90's based on lithium insertion electrodes essentially the high voltage cobalt oxide cathode invented by J. B. Goodenough and the carbon anode using coke or graphitized carbonaceous materials.
Since then, lithium-ion batteries have progressively replaced existing Ni—Cd and Ni-MH batteries, because of their superior performances in most portable electronic applications. However, because of their cost and intrinsic instability under abusive conditions, especially in their fully charged state, only small cell size and format have been commercialized with success.
In the mid 90's, Goodenough (See U.S. Pat. Nos. 5,910,382 and 6,391,493) suggested that polyanionic phosphate structures, namely nasicons and olivines, could raise the redox potential of low cost and environmentally compatible transition metals such as Fe, until then associated to a low voltage of insertion. For example LiFePO4 was shown to reversibly insert-deinsert lithium ion at a voltage of 3.45 V vs a lithium anode corresponding to a two phase reaction. Furthermore, covalently bounded oxygen atom in the phosphate polyanion eliminates the cathode instability observed in fully charged layered oxides, making an inherently safe lithium-ion battery.
As pointed out by Goodenough (U.S. Pat. No. 5,910,382 & 6,514,640), one drawback associated with the covalently bonded polyanions in LiFePO4 cathode materials is the low electronic conductivity and limited Li+ diffusivity in the material. Reducing LiFePO4 particles to the nanoscale level was pointed out as one solution to these problems as was the partial supplementation of the iron metal or phosphate polyanions by other metal or anions.
One significant improvement to the problem of low electronic conductivity of complex metal oxide cathode powder and more specifically of metal phosphate was achieved with the use of an organic carbon precursor that is pyrolysed onto the cathode material or its precursor to improve electrical field at the level of the cathode particles [Ravet (U.S. Pat. Nos. 6,963,666, 6,855,273, WO/0227824 and WO/0227823)].
It is also known to improve conductivity of a phosphate powder when used as a cathode material, by intimately mixing conductive carbon black or graphite powder with the phosphate powder or the phosphate precursors before synthesis Such addition of carbon blake or graphite powder involves usually relatively large quantities of C to achieve good connectivity and does not result in a good attachement of the C to the metal phosphate crystal stucture, said attachment being a characteristic judged essential to maintain contact despite volume variations during long term cycling.
Such recent improvements have led several battery manufacturers and users to undertake the development of safe mid to large size lithium-ion batteries based on transition metal phosphates cathode materials for use in portable power tools, Hybrid Electric Vehicle (HEV) and Plug-in HEV as well as for large stationary batteries for backup power and energy storage from intermittent sources.
Problems remain however to optimize the processability, cost and performance especially when power, energy arid cyclability are required simultaneously.
Composite electrode optimization, for example, requires short distances for Li+ diffusion in the solid state and the presence of an electronically conductive phase at the level of each nanoparticle of LiFePO4. Manipulation and processing (coating and compacting) of elementary nanoparticles or their dispersion is more complex than manipulation and processing micron-size particles, given their large surfaces and low compaction. In the present text, nanoparticie means a particle having dimensions ranging from 5 nm to submicrons (defined as less than 1.0 μm), preferably between 20 and 600 nm that can be primary or secondary particles. A primary particle is made of a complex oxide. A secondary particle is an aggregate of primary particles, and may also contain other constituants such as internal or external C-deposit or carbon bridging or particulate carbon, other inert or conductive phases or sintering necks. A secondary particle may also have a porosity.
The present inventors found that the use of agglomerates of primary and secondary nanoparticles which are elaborated at a micron-size scale or larger (by spray drying for example), instead of elementary nanoparticles as such, facilitates ions and electron diffusion and the electrochemical reaction This is the result of using nano dimensions at the level of the active material nano particles while, benefiting from the ease of manipulating micron-size agglomerates.
As a general rule, electrochemical performance optimization of such agglomerates of nanoparticles or nanocomposite material requires a material having a high proportion of active metal phosphate, a low proportion of electrochemically inert conductive carbon and a controlled degree of open porosity of the agglomerates or the nanocomposite material. Furthermore, pore channel dimensions must be designed to allow solvated lithium's ion of the electrolyte to penetrate and reach elementary nano sized particles to support high charge or discharge rate currents.
Designing such agglomerates of nanoparticles or nanocomposite materials, as well as attaching efficiently nanolayers of conducting carbon to single or agglomerated nano particle internal or external surface becomes a challenge in order to avoid using too much dead weight carbon. The present invention addresses this problem at the level of presynthetised transition metal phosphates as well as at the level of the metal phosphate precursors.
It is known that metal phosphate agglomerated precursors have great impact on the structure and the properties of lithium metal phosphate final product (WO/0227824 and WO/0227823). For example, most of the commercially avai-lable FePO4, 2H2O which is a precursor for LiFePO4, is prepared by a wet chemistry process and has large dense aggregates having a mean particle size in the range of 40-200 μm and composed of fine elementary particles having a mean particle size in the range of 0.1-1 μm. Synthesis of lithium metal phosphate using large agglomerated particle precursors requires long sintering times and sometimes leads to large particle size, sintered material and impurity phases due to incomplete reaction between the reactants.
Pre-synthesis grinding of FePO4.2H2O by jet milling can reduce the size of secondary particles to micron size, for example D50 at 2 μm and DIOO at 10 μm. The electrochemical performance of a carbonated Li—Fe-phosphate (designated by LiFePO4/C) can be significantly improved by using air jet milled FePO4.2H2O precursor. However, sintering still occurs inside the large agglomerates and leads to limited power capability of an electrode made of said LiFePO4/C.
It is known that when organic carbon precursors are used in the process for preparing Li metal phosphate materials, the non-agglomerated nanosize FePO4.2H2O particles used as the precursor remain un-agglomerated even at the optimized synthesis temperatures required to obtain lithium metal phosphate. In contrast, dense or close porosity large particles made of agglomerates or aggregates tend to sinter to a large degree even when an organic precursor is used (WO/0227824 and WO/0227823). Such dense and large particles made of agglomerates or aggregates lower the rate performance of the final products because of low Li+ diffusion and/or lack of conductive carbon inside the particles.
It is therefore a critical step to prepare the metal phosphate precursor so as to achieve non agglomerated and well dispersed fine particles in the nanometer and sub-micron range before sintering synthesis. In another aspect of the invention, it is also possible to create precursor agglomerates having the right structure, porosity and carbon precursor localization from said well dispersed nano particles in order to design optimized agglomerates of nanoparticles or nanocomposites of the final product. There are many ways and technologies available to obtain non-agglomerated tine particles depending on the physical properties of the available metal phosphate. For examples, if the metal phosphate is not made of hard agglomerates or aggregates, ultrasounds can be used to break the secondary particles and disperse the elementary particles or smaller agglomerates and stabilized the liquid suspension of those by using and organic stabilizer or dispersant. Grinding or comminuting is one of the most used processes allowing the production of fine particles and/or to de-agglomerate. More recently, industrial ultra fine wet grinding equipment have been made available commercially that can be used to reduce particle size down to 10 to 20 nm. However, with time the nano particles tend to re-agglomerate due to strong van der Waals interaction or electrical double layer interaction.
Various processes have been used to make lithium metal phosphate or carbon-coated lithium metal phosphate materials. One of them is solid state reaction of various precursors under reducing or inert atmospheres. Depending on the nature and particle size of the reactants, various reaction temperatures and times are required to achieve high purity lithium metal phosphate. In most cases, the reaction temperature required to achieve complete reaction is high and is accompanied with sintered aggregates or sintering necks.
Wet chemistry methods like co-precipitation and sol-get synthesis have been widely investigated to make homogeneous sintering precursors at atomic scale and in principal, a low pyrolysis temperature is needed to achieve fine particle size of final products. However, in practice, a segregation of reacting species occurs, and then long reaction times or higher reaction temperatures are required to achieve high crystallinity and high phase purity. In consequence, the particle size and particle morphology are complex to control.
Hydrothermal reaction is one of the most elegant methods to synthesize lithium metal phosphate. The lithium metal phosphate particles with various well controlled particle sizes and morphologies under moderate hydrothermal conditions can be made. Depending on the precursors and hydrothermal conditions, various particle size and shapes have been reported such as submicron size ellipsoids, micron size hexagonal plate and heavily agglomerated nanospheres or nano-rods. Difficulties are often associated with the control of stoichiometry, crystallinity, phase purity and particle size.
In many of the processes reported so far, difficulties associated with the control of particle size, phase purity and carbon coating are the bottleneck to scale up the process. To avoid abnormal particle growth, a low sintering temperature is required. On the other hand, to achieve high phase purity and high carbon conductivity, a higher sintering temperature is desired. It is difficult to achieve all optimized parameters in one single synthesis step.
In an earlier work the applicants have also developed a low cost synthesis process to prepare a phosphate cathode material which has been patented (See WO 2005/062404) but said process results in solid crystalline ingots or micron size powders as made by conventional grinding process.
Grinding or comminuting is one of the most used processes allowing the production of fine particles and/or to de-agglomerate in ceramic and paint industries. More recently, industrial wet nanogrinding bead mill equipment have been made available commercially, that can be used to reduce particle size down to 10 to 20 nm (See for example WO 2007/100918 for lithium metal phosphate ultrafine grinding).
During wet nanogrinding in isopropyl alcohol solvent, preliminary experiments on pure LiFePO4, obtained from a melt process, inventors were drawned to conclusion that such mechanical treatement present deleterious effects that affect the use of said pure LiFePO4 as a cathode material. Indeed, after nanogrinding LiFePO4 in the range of 20-30 nm, only a 4% reversible capacity was realized in a lab-cell instead of the expected >80% as shown and discussed in a following example. After that point, it was concluded that wet nanogrinding a lithium metal phosphate was altering the product. However, when nevertheless a batch of this nanoground LiFePO4 was subsequently heat treated and used for a pyrolysis carbon-deposit experiment the inventors surprinsigly discovered that electrochemical properties of such carbon-deposit LiFePO4 were restored as 94% of the reversible capacity was realized. This unexpected effect of a deterioration of pure LiFePO4 by wet nanogrinding followed by restauration of electrochemical properties through thermal treatment and carbon deposition by pyrolysis is a main object of the present invention as well as the use of different organic surfactants, adsorbant and carbon precursors that facilitate the wet grinding process and that are converted to non contaminating and conductive carbon to make and optimize the C deposited nano particle or nanostructured lithium metal phosphates particle or agglomerates.
The present invention provides a method for preparing carbon-deposited cathode nano materials, including a from molten lithium metal phosphates process and ingots in an easy way and resulting in a high performance cathode material.